Has anyone had this happen? This past weekend I noticed 2 pinholes in the lower 2-3 feet of the mast (black anodized) of our 1989 Hobie 18. Very strange, and there is no pitting or other obvious corrosion to be seen. I sealed them with silicon to keep water out, but I was wondering if anyone has any idea what could cause this.

Are you the original owner of the boat? If not, has the boat been sailed in salt water? Not knowing where you are located, could the mast got water in it and froze? I'll assume these holes are not drilled by a previous owner to attach an accessory. Just a few things to ponder.

We're not the original owners, but the boat was 3 years old when we got it. Always sailed in salt water, and the holes just appeared (as far as i know) this season, not really close to any fittings, but within a few inches of the downhaul fitting and the mast rotatino arm, and they are not drilled: too small and too irregular.

I agree, my first thougt would be salt water trapped inside the mast causing internal corrosion. The bottom mast plug is about 1 to 2 feet up from the mast base. If you store your boat with the mast up and salt water is allowed to collect above the bottom plug, it's eventually going to corrode the aluminum.

You may want to consider removing the mast base and bottom plug and inspecting the inside of the mast. If it's badly corroded, you won't want to be sailing it.